1. Field of the Invention
The present invention relates to a device for detecting the knocking of an internal combustion engine by judging the knocking occurring in an internal combustion engine relying upon a change in the level of an ionic current flowing through an ignition plug during the combustion in an internal combustion engine. More particularly, the invention relates to a device for detecting the knocking of an internal combustion engine, which, in judging an integrated value of knock signals, excludes noise components of small amplitudes that are lasting long, in order to improve an S/N ratio of the integrated value and to reliably prevent such an occurrence that noise is detected as knocking.
2. Prior Art
In an internal combustion engine as is well known, a mixture of the air and the fuel introduced into a combustion chamber is compressed by the rise of a piston, an electric spark is generated by applying a high voltage to a spark plug installed in the combustion chamber to burn the mixture, and the force produced as the piston is pushed down is recovered as an output.
When the combustion takes place in the combustion chamber, molecules in the combustion chamber are ionized. When a high voltage is applied to the spark plug (electrode for detecting ionic current) installed in the combustion chamber, a current (ionic current) flows due to ions having electric charge.
The ionic current sensitively changes depending on a change in the pressure in the combustion chamber. It has therefore been known that the ionic current includes a vibration component corresponding to the amount of knocking.
It is therefore allowed to judge the occurrence of knocking relying on the ionic current.
There have heretofore been proposed a variety of devices for detecting the knocking of an internal combustion engine by judging the occurrence of knocking relying on a change in the amount of ions produced by the combustion in the internal combustion engine. For example, there has been proposed a device for judging the knocking relying on a peak-holding value of knock signals picked up from the ionic current.
When the peak-holding value is used for judgment, however, the S/N ratio drops being affected by a stepwise change in the ionic current resulting in an erroneous judgement. In order to suppress the effect caused by an instantaneous change in the ionic current, therefore, there has been proposed a device that judges the knocking relying on an integrated value of knock signals.
FIG. 4 is a block diagram schematically illustrating major portions of a conventional device for detecting the knocking of an internal combustion engine disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 159129/1994, and shows only a circuit for operating an integrated signal Km for judging the knocking.
In FIG. 4, an ionic current detecting circuit 1 related to an ignition device (not shown) includes a bias current source for feeding an ionic current through an ignition device, and amplifies and detects the ionic current, and outputs an ionic current detection signal Ei.
The ionic current detection circuit 1 may be constituted in any manner provided it finally outputs knock signals Ki in the knocking frequency band.
As is well known, the bias current source is connected to the spark plug though not diagramed here.
A band-pass filter 2 has predetermined frequency characteristics corresponding to knocking vibration, and picks up knock signals Ki in an amplified manner from the ionic current detection signals Ei.
An absolute value circuit 3 produces the knock signal Ki picked from the ionic current detection signal Ei as an absolute value.
An integrating circuit 4 integrates the knock signal Ki in the form of an absolute value to produce an integrated signal Km.
The integrated signal Km is input to a comparator means 5 (knock-judging means), compared with the background level (knock-judging level), and is used for judging the occurrence of knocking.
FIG. 5 is a diagram of waveforms illustrating the operation of the conventional device for detecting the knocking of an internal combustion engine.
In FIG. 5, an ignition signal P is applied to an ignition device for every ignition cycle of the internal combustion engine, turns the power transistor (not shown) on and off as is well known, and interrupts the flow of current into the primary winding (not shown) of the ignition coil.
The ionic current detection signal Ei is formed immediately after the ignition signal P is turned on and during the combustion period after the ignition signal P is turned off.
The knock signals Ki are formed by picking up the high-frequency vibration components only superposed on the ionic current detection signal Ei.
The integrated signal Km holds the peak by integrating the absolute value components of the knock signals Ki, and is reset after every data-fetching timing t1, t2.
In, for example, the integrated signal Km, the waveform Kma at the time when the knocking has occurred is reaching a level larger than the background level BG. When the noise vibration Kn has occurred having a small amplitude and lasting long, the waveform Kmb of the integrated signal has a level lower than that of the waveform Kma of when the knocking has occurred but is still relatively high.
In FIG. 5, even during the normal combustion in which no knocking is occurring, the noise vibration of a small amplitude is superposed on the ionic current detection signal Ei, and the noise vibration Kn of a low level may often be detected as a knock signal Ki.
Further, due to a stepwise change in the ionic current detection signal Ei, a peak waveform of the same level as that of when the knocking has occurred is often superposed on the knock signal Ki.
The stepwise current that changes within very short periods of time little affects the integrated value, and is smaller than the integrated value of when the knocking has occurred, making it possible to maintain a correct knocking detection.
However, when the noise vibration Kn has occurred having a small amplitude and lasting long, no large difference is produced between the waveform levels Kma and Kmb of the integrated signals corresponding to the presence and absence of knocking, and it becomes difficult to detect the knocking.
In particular, noise vibration Kn of a frequency near the knock frequency band is often superposed on the ionic current detection signal Ei due to a change in the power source voltage or ground potential, being affected by electromagnetic waves or due to a change in the ionic current itself.
In such a case, the integrated signal Km rises due to noise components of small amplitudes lasting long, making it very difficult to maintain an S/N ratio for favorably judging the knocking.
In the conventional device for detecting the knocking of an internal combustion engine, when the noise vibration Kn of frequency near the knock frequency band having a small amplitude and lasting long is superposed on the ionic current detection signal Ei, the integrated signal Km rises above the background level BG, and noise may be erroneously judged as the knocking.